There exist any number of plant diseases which are caused by viruses, among them Tobacco Mosaic Virus (TMV), Cucumber Mosaic Virus (CMV), Potato Virus X (PVX) and Potato Virus Y (PVY), which cause damage to plants of commercial significance such as tobacco, tomato, sweet and bell peppers and so on. It is, therefore, desired to find a way to immunize these plants against such viruses.
Viruses spread throughout a plant in two stages. First is infection wherein the virus in introduced into the cells of the plant, and the second is replication wherein the virus muliplies within the cells of the host plant.
Plants have several natural defense mechanisms against the spread of viruses. One of these is the phenomenon of local lesion infection whereby the virus is confined to the lesion and replicates only a small group of cells around the point of entry. Another is the so-called "green islands" phenomenon, an uneven distribution of virus often seen in plants having systemic infection. These are dark green areas which generally contain little or no virus and are often resistant to reinfection. The mechanisms of these defense mechanisms are not understood.
Methods of preventing virus increase in plants in use today are generally aimed at the first stage of virus development, that is, preventing infection of the plant. Thus, there are on the market many materials which, when mixed with purified solutions of virus, prevent infection when applied to other plants. Other chemicals, for example, nucleic acid analogues, and inhibitors of nucleic acid or protein synthesis, are available which inhibit the replication of viruses in the laboratory but these have phytotoxic effects which have a significant detrimental effect on the plants as well as on the viruses.
Thus, there exists at present no practical chemotherapeutic system for supression of virus replication in plants.